The Sittar poetry of Tayaumanavar became famous because his writings were protests against the caste system. Tayaumanavar, a prominent Tamil poet and philosopher from the 18th century, expressed his discontent and opposition towards the prevailing caste-based discrimination and social hierarchy through his poems.

Option 1, which suggests that the writings were nationalist compositions, is incorrect as the focus of Tayaumanavar`s poetry was on social issues rather than nationalism.

Option 2, which states that the writings were romantic epics, is also incorrect as Tayaumanavar`s poetry did not revolve around romantic themes.

Option 3, which suggests that the writings were devotional songs, is partially correct but does not capture the main reason for the popularity of his poetry. While Tayaumanavar did compose devotional songs, his writings gained fame primarily due to their strong critique of the caste system.

Therefore, option 4 is the correct answer as it accurately describes the reason for the fame of Tayaumanavar`s Sittar poetry.